1. Field of the Invention
This invention pertains to an improved screwdriver having a single drive shaft rotatably mounted in a one-piece handle member, reduction gearing arranged therebetween, and lock means mounted in the handle member; the lock means being operable, in one position, to lock the handle member to the drive shaft whereby the reduction gearing is bypassed for operation like a conventional screwdriver, and in another position, to permit rotation of the handle member relative to the drive shaft whereby the reduction gearing is operable to increase the torque applied to a screw being turned.
2. Description of Prior Art
A screwdriver, in most common form, comprises a single drive shaft a rearward portion of which is rigidly fixed to a one-piece solid plastic handle and a forward portion of which is shaped to engage a slot-head, philips-head or other type of screw. In another form, the forward portion of the drive shaft is adapted to detachably engage alternative screwdriver bits which differ according to the type of screw head that is to be turned. In either embodiment, the drive shaft turns the same number of degrees as the handle and so the amount of torque that can be applied to the screw is limited according to the strength of the user.
When installing a woodscrew, depending upon the length and diameter of the screw, the hardness of the wood and the size of a pilot hole, if any, substantially increased resistance may be experienced during the last few turns of the screw or during the first few turns when removing such screw. Unexpected resistance may also be experienced when attempting to force a self-tapping screw through a pilot hole that is a little too small. Of course, even for woodscrews, one should provide a pilot hole of correct size, but oftentimes, even when it would seem one has done so, significant resistance may be encountered. At such times, common practice is to grip the screwdriver handle as tightly as possible and force it to turn. This is quickly tiring and even painful, depending upon the size of the screw and how many are being installed or removed. When it can be done without splitting the wood or, in the case of a self-tapping screw, without stripping the screw, a longer screwdriver can be used to apply greater turning force to the screw, but only a moderate mechanical advantage is gained thereby. Many households are fortunate to possess a single screwdriver, typically of medium size, let alone an assortment including an extra-long screwdriver. Even the well-equipped mechanic does not always have exactly the right screwdriver close at hand when needed.
There has long been need for a screwdriver that is adjustable to select an alternative operating mode whereby a greater torque can be applied to the screw than the user is applying to the handle; but, not costing significantly more than a conventional screwdriver, and, without loss of any of the advantages provided by a conventional screwdriver, such as simplicity for durability, compactness and lightness of weight for carrying on one's person or in a tool belt, and an unobstructed view of the screw being turned.
The nearest prior art known comprises U.S. Pat. Nos. 2,721,591, 3,992,964, 4,048,874. 4,846,027 and 5,033,336, which disclose screwdrivers and devices similar thereto wherein gearing is arranged between a handle and an output shaft in order to effect alternative drive ratios therebetween, but in every instance, the disclosed device comprises so many parts that a screwdriver so constructed would cost several times as much as a conventional screwdriver, the price of which determines what the consumer expects to pay for a screwdriver. No prior art is known to disclose a reduction geared screwdriver having such simplicity that it could be manufactured to sell at a price comparable to a conventional screwdriver; which may explain why one does not find reduction geared screwdrivers on display where hand tools are sold and the public has continued to be denied the benefit of such a screwdriver.
The referenced prior art is limited by comprising numerous components including one or more of the following: internally toothed gears cooperating with planetary gears, plural drive shafts or complex linkages ultimately defining a drive shaft, and a handle that either is disclosed as comprising more than one part, or, clearly could not be manufactured to incorporate lock means therein except by construction in plural parts. A further limitation characterizing the referenced prior art is that the reduction gearing is always supported by and concealed within an enclosing cylindrical housing (other than the handle) that, having a diameter greater than that of the handle, further obstructs (beyond that of the gearing itself) the user's view of the screw that is being turned. Reliance upon a housing to support the gearing leads to the further limitations of added weight and bulk, and, because frequently comprising plural parts, to added cost. In order to compete successfully against conventional screwdrivers, a reduction geared screwdriver must be characterized by comparable simplicity for comparable cost, compactness, ruggedness and lightness of weight.
U.S. Pat. No. 3,992,964, listed above and which is entitled "Torquing or Speeding Lug Wrench" is believed to be the closest approach of the prior art. It discloses what is therein termed a "gear box" in the form of a two-part cylindrical housing having parallel spaced apart first and second walls which close opposed ends of the assembled cylindrical housing. A first drive shaft is supported solely by and rotatably secured to the first wall, and a second drive shaft is supported solely by and rotatably secured to the second wall; the two shafts appearing as one because arranged end-to-end, but in fact being distinct members which are rotatable relative to each other. Either one of the shafts may serve as a handle (by connecting a detachable cross-bar thereto) when the other shaft is serving as an output or drive shaft (by connecting a detachable lug-engaging fitting thereto), depending upon whether the device is being used to turn a lug faster (the speeding mode) or to turn a lug with increased torque (the torquing mode). Alternative embodiments are disclosed, one of which, illustrated in FIG. 2 of such patent, shows four gears which are functionally related to each other in the manner of the invention disclosed herein, at least when used in the torquing (or reduction gear) mode. The arrangement disclosed in such patent cannot produce a reduction geared screwdriver comparable in cost and simplicity to a conventional screwdriver or having the other benefits sought to be provided. There are numerous differences, only three of which are pointed out here. First, two drive shafts are involved, instead of one drive shaft as in the present invention. Second, the two drive shafts are independently supported by and independently secured to the gear box; as opposed to the present invention wherein a single drive shaft is supported solely by and rotatably secured to a handle. Third, the use of two, independently rotatable drive shafts, requires that the gear box be held stationary (non-rotatable) during use of the device. For that purpose, the disclosure provides an "extension brace 16" fixed to and extending laterally from the one side of the gear box in order that the free end of such brace will abut the ground and thereby prevent rotation of the gear box when the shaft being used as the handle is being turned. Accordingly, the transfer gears, which are rotatably mounted within and supported by the gear box, have their pivotal axis held stationary with the gear box while a lug is being turned. The present invention provides its transfer gears rotatably mounted and exposed on a pivot that is fixed to the free end of a lug member that revolves with the drive shaft as the screw is being turned; this mode of support of the transfer gears gaining the advantage of minimally obstructing the user's view of the screw being turned, as well as reducing cost, bulk and weight.